In the reproduction of copies of an original from video image data created, for example, by electronic input scanning from an original document, it is often desirable to provide functions dependent on determining the exterior edges of the document. Such features include, for example, automatic magnification, automatic two-up copying, background deletion to avoid storing a document requiring more space than necessary, etc. In the analogous field of light lens copying this has commonly been done by providing a background, against which the document is viewed, that has a tendency to be distinguishable from the image. Thus, for example, backgrounds have been provided with a yellow color, with a black color, with a white color with a whiter than white color, and various other alternatives. All these alternatives work to some extent, but each suffers from the problem that for some documents the background is not well distinguishable from the sheet edge. Thus, black edges of a sheet cannot be distinguished against a black background, canary paper may not be distinguishable against a yellow background, white paper on a white background is questionably detectable, etc.
Various methods are known for distinguishing types of images in a document based on the frequency of the image data. Thus, for example, in U.S. Pat. No. 4,194,221 to Stoffel, this problem was addressed by applying a discrimination function instructing the image processing system as to the type of image data present and particularly, an auto correlation function to the stream of pixel data, to determine the existence of high frequency half-tone image data.
Stoffel describes a method of processing automatically a stream of image pixels representing unknown combinations of high and low frequency half-tones, continuous tones, and/or lines to provide binary level output pixels representative of the image. The described auto correlation function is applied to the stream of image pixels and, for the portions of the stream that contained high frequency half-tone image data, notes a large number of closely spaced peaks in the resultant signal. GB No. 2,153,619A provides a similar determination of the type of image data. However in that case, a threshold is applied to the image data at a certain level, and subsequent to thresholding the number of transitions from light to dark within a small area is counted. The system operates on the presumption that data with a low number of transitions after thresholding is probably a high frequency half-tone or continuous tone image. In U.S. Pat. No. 4,811,115 to Lin et al, a method and apparatus for applying an auto correlation function for the determination of the presence of half-tone image data is shown. The auto correlation function is calculated for the stream of half-tone image data at selected time delays which are predicted to be indicative of the image frequency characteristics, without prior thresholding. The arithmetic function used in the auto correlation system of U.S. Pat. No. 4,811,115 to Lin is an approximation of the auto correlation function using logical functions and addition, rather than the multiplication function used in U.S. Pat. No. 4,194,221 to Stoffel. Peaks in the resulting auto correlated function are detected to determine whether half-tone image data of selected frequencies are present.
Of interest to the problem of background detection are: U.S. Pat. No. 4,338,020 to Yukawa et al. which in an electrophotographic device provides a colored document background and an arrangement for optimizing detection thereof, U.S. Pat. No. 4,511,246 to Nishiyama, U.S. Pat. No. 4,568,181 to Nishiyama, and U.S. Pat. No. 4,713,550 to Anzai et al. show, in an electrophotographic device, a series of spaced light detectors which detect light reflected from a background to determine which of a series of paper sizes is registered at an edge position on a platen. U.S. Pat. No. 4,415,261 to Yukawa et al. provides in an electrophotographic device a detectable marking on the interior surface of a platen cover with fluorescent paint or pigment. The markings are detectable by an undisclosed sensor. U.S. Pat. No. 4,541,713 to Maekawa teaches in an electrophotographic device that a document size detecting arrangement may be provided with a platen cover having a plurality of colored portions, with a light receiving arrangement adapted to distinguish the colored portion from the rest of the surface. U.S. Pat. No. 4,630,127 to Fuwa teaches a platen cover with two sets of linear arrays of light sources in main and sub scanning directions, and a sensor for detecting the light sources, detection of given number indicating document size. U.S. Pat. No. 4,620,781 to Miyamoto teaches the use of a specularly reflective background surface, and the use of a sensor positioned to receive light scatterably reflected by the original document, but not light specularly reflected by the background, to allow detection the edges of the document. U.S. Pat. No. 4,260,248 to Murata et al teaches adjustment of the lens focus, based on known copy paper size and amount of magnification desired. U.S. Pat. No. 4,746,953 to Knodt teaches data input regarding sheet size with touch sensitive switches along the edges of the platen. U.S. Pat. No. 4,391,505 to Silverberg teaches lead edge registration of a document on a transport base on occlusion of a spectrally reflective patch, and U.S. Pat. No. 4,243,925 to Gnucehtel teaches registration of output sheets in a input scanning device by counting encoder revolutions. All of the above-mentioned patents are incorporated by reference for their teachings.